The present invention relates to vacuum servomotors designed in order to apply an additional force to an output member when a force is applied to an input member bearing on the output member and/or in order to apply a force to an output member following a signal emitted by a device outside the servomotor. Such servomotors are used on motor vehicles, particularly in order to boost braking.
Such a vacuum servomotor conventionally comprises a main casing divided sealingly on the inside into first and second chambers by a flexible membrane bearing on a rigid disk integral with an activating piton. The first chamber is subjected to a low pressure whilst the second chamber is subjected either to this low pressure when the servomotor is at rest, or to a higher pressure when the servomotor is in operation.
These servomotors have a large size which increases with the boost required. In point of fact, the engine compartment of present-day motor vehicles is extremely full and it is sometimes impossible to dispose, in this compartment, a servomotor whose outer diameter corresponds to the power boost required by the manufacturer.
Some manufacturers therefore use a servomotor of small size, the boost required then being obtained either by connecting the first chamber to a vacuum source which is greater than the usual source, or by subjecting the second chamber to a pressure which is greater than atmospheric pressure. In all cases, an auxiliary pump is required and this substantially increases the overall cost price of the boost device.